The present invention relates to a flow-sensing device and method for fabrication and more particularly, relates to a flow-sensing device that utilizes non-platinum materials for the sensing elements which can be fabricated by a thick film printing process and a method for such fabrication.
In the design and operation of internal combustion engines, the minimization of exhaust emissions and the improvement in fuel efficiency are two important aspects that must be fulfilled. These aspects can be accomplished, partially, by maintaining an optimum air-to-fuel ratio during the internal combustion process. The optimization of the air-to-fuel ratio in turn requires accurate measurements of an airflow rate into the internal combustion engine. Conventionally, the measurements are accomplished by using a mass airflow sensor (MAFS).
A mass airflow sensor is typically constructed of temperature sensing elements mounted in a rigid housing to form a part of the airflow chamber between an engine air filter and the manifold. The temperature sensing elements which are the heart of the sensor are generally housed in a smaller plastic compartment located on top of the rigid housing formed of either metal or plastic. The sensing elements consist of two elements suspended within the housing such that they are exposed to an airflow through the mass airflow sensor. The material and process for forming the sensing elements are therefore critical in the proper design and operation of the mass airflow sensor circuitry.
The design and fabrication of the sensing elements are important factors in achieving mass airflow sensors that are capable of accurate measurements. For instance, the sensing elements must typically have not only a high temperature coefficient of resistance (TCR), but also a consistent TCR value among the sensing elements fabricated so that they can be paired together. To achieve such consistency, deviations in resistance ratios of the sensing elements at selected ambient temperatures must be minimized. It has been found that in order to properly minimize the resistance ratio deviations, it is desirable to have sensing elements that have resistance values between 19.5 and 20.5 ohms at room temperature, and a difference between the resistances of the elements that are paired together to be less than 0.1 ohm.
U.S. Pat. No. 5,508,491, assigned to the common assignee of the present invention, discloses an electronic mass airflow sensor circuit and a method for manufacturing in which the use of a thick film resistor in an analog conditioning circuitry, rather than as the sensing elements, is disclosed. The circuit utilizes a thick film resistor electrically connected to a secondary sensor element to form a combination element such that resistance ratio of the primary sensor element to the combination element remains constant over varying temperatures.
U.S. Pat. No. 4,735,086, assigned to the common assignee of the present invention, discloses a mass airflow meter that has a first electrically heated resistive film and a temperature dependent resistive film sandwiched between plastic layers and positioned within an air duct. Symmetrically positioned around the first resistive film are a pair of electrically heated resistive films and associated temperature sensors, also encapsulated within plastic layers. Electronic circuit is used to apply electric power to the resistive films for maintaining each of them at a predetermined temperature above ambient air temperature regardless of airflow.
U.S. Pat. No. 5,251,481 discloses a method for protecting a temperature dependent resistor layer from dust by using a special two-layer coating and by placing the resistor pattern recessed from the leading edge where deposits most frequently occur. A material for the temperature dependent resistor layer in the surface resistor of a thin film sputtered platinum applied to a large area and then laser trimmed to shape is disclosed.
U.S. Pat. No. 5,351,536 discloses a finger-type design in which the sensing elements and the conditioning circuitry are placed on the same substrate which is of an irregular, elongated shape so that the temperature sensing element projects into the air stream and the conditioning circuit remains protected from the air stream. While it is mentioned that the resistors can be fabricated by either a thick film or a thin film technique, no processing or design details were disclosed.
In order to fabricate sensing elements that have consistent resistance values, the elements should be fabricated in the same process so that variances caused by changing processing parameters can be eliminated. Furthermore, to lower the cost of fabrication for the sensing elements, alternative non-platinum materials that have similar TCR values as platinum are also desirable.
It is therefore an object of the present invention to provide a gaseous flow sensor that does not have the drawbacks or shortcomings of the conventional gaseous flow sensors that utilize platinum sensing elements.
It is another object of the present invention to provide a gaseous flow sensor that contains sensing elements fabricated of a material that has a consistent temperature coefficient of resistance value.
It is a further object of the present invention to provide a gaseous flow sensor that contains sensing elements that are fabricated of a low cost, non-platinum material.
It is another further object of the present invention to provide a gaseous flow sensor that utilizes sensing elements that are fabricated of a ruthenate-based material by a thick film printing process.
In accordance with the present invention, a gaseous flow sensor and a method for fabricating such sensor are provided.
In a preferred embodiment, a gaseous flow sensor is provided which includes a substrate that is formed of an electrically insulating material; a reference resistor formed on the substrate and disposed in the gaseous flow without heating at an ambient temperature; a flow-sensing resistor formed on the substrate and disposed in the gaseous flow heated to a temperature higher than the ambient temperature, wherein the reference resistor and the flow-sensing resistor are formed of a non-platinum resistive material; and an electrical circuit in electrical communication with the reference resistor and the flow-sensing resistor.
In the gaseous flow sensor, the non-platinum resistive material includes an oxide composition of Pb, Ru, Si and Bi. The reference resistor has an electrical resistance that is at least 15 times the electrical resistance of the flow-sensing resistor. The reference resistor and the flow-sensing resistor each has a thickness between about 2 xcexcm and about 30 xcexcm, and preferably a thickness between about 5 xcexcm and about 20 xcexcm. The reference resistor may be formed in a serpentine configuration, or formed in a serpentine configuration that has vertical portions connected by horizontal portions with an aspect ratio of length/width of the resistor being at least 2. The electrical circuit maintains a target temperature differential between the reference resistor and the flow-sensing resistor by controlling the magnitude of an electrical current flowing to the flow-sensing resistor. The reference resistor may also be formed in a spiral configuration.
The present invention is further directed to a gaseous flow sensor that includes a substrate that is formed of an electrically insulating material; a reference resistor that is formed on the substrate and disposed in the gaseous flow without heating at an ambient temperature; a flow-sensing resistor formed on the substrate and disposed in the gaseous flow heated to a temperature higher than the ambient temperature, wherein the reference resistor and the flow-sensing resistor both are formed of a single non-platinum resistive material; and an electrical circuit in electrical communication with the reference resistor and the flow-sensing resistor.
The present invention is still further directed to an airflow meter that includes an insulating substrate; a first resistor formed on the insulating substrate that has a first resistance, the first resistor is maintained at ambient temperature; a second resistor formed on the insulating substrate that has a second resistance, the second resistance is maintained at a temperature higher than the ambient temperature, while the first resistance is at least 15 times the value of the second resistance; and an electrical circuit for comparing the second resistance to the first resistance.
In the airflow meter, the insulating substrate may be formed of a ceramic material. The first resistor may be formed in a serpentine configuration, or in a serpentine configuration that has an aspect ratio (length/width of resistor) of at least 2. The first and second resistors may be formed in a thickness between about 2 xcexcm and about 30 xcexcm, and may be formed of a non-platinum containing resistive material.
The present invention is still further directed to a method for fabricating a gaseous flow sensor that can be carried out by the operating steps of first thick film printing a reference resistor from a non-platinum containing paste; thick film printing a flow-sensing resistor from a non-platinum containing paste; forming a circuit for providing electrical communication between the reference resistor and the flow-sensing resistor and for determining a differential resistance therein-between.
The method for fabricating a gaseous flow sensor may further include the step of forming the reference resistor and the flow sensor resistor in the same thick film printing process. The method may further include the step of forming the reference resistor with a resistance that is at least 15 times the resistance of the flow-sensing resistor. The method may further include the step of thick film printing the reference resistor and the flow-sensing resistor to a thickness between about 4 xcexcm and about 50 xcexcm, or the step of firing the reference resistor and the flow-sensing resistor after the thick film printing step.